In certain electronic applications it is necessary to measure the value of the current in the inductors with good accuracy and while disturbing as little as possible the device in which the inductor is inserted. We may cite for example the measurement of the current in the inductors of switched mode power supplies, among other things, voltage step-down or step-up switched mode serial choppers (respectively known as “buck converters” and “boost converters”).
The conventional method which is most used consists in placing a measurement shunt in series with the inductor, thereby making it possible to obtain the image of the current in the inductor by measuring the voltage across the terminals of the shunt. This solution has the advantage of being simple to implement, on the other hand it has a major drawback. Specifically, the power dissipated in the shunt contributes to a degradation in the overall efficiency of the converter, which is not always acceptable, this being all the more true when the output voltage of the energy conversion device is low. To avoid this constraint, a non-dissipative solution for measuring current, presented in FIG. 1, is used.
FIG. 1 shows a diagram of a device 10 for measuring the current I passing through an inductor 12. The inductor 12 is represented by its equivalent diagram comprising a pure reactive part, i.e. the inductive part L in series with a resistor RL. The inductor comprises a terminal A and a terminal B.
The inexpensive and very non-dissipative measurement device 10 is placed in parallel with the terminals A and B of the inductor 12. The measurement device 10 comprises a resistor R2 in series with a resistor R1 in parallel with a capacitor C1, the resistor R2 being connected to the terminal A, the resistor R1 to the terminal B.
The aim of this arrangement of the state of the art is to obtain a voltage across the terminals of the capacitor C1 proportional to the voltage across the terminals of the resistor RL of the inductor 12 hence proportional to the current I in the inductor 12 (or in the inductor L). The currents in the resistors R2 and R1 are negligible compared with the current I in the inductor 12.
To size the elements of the device, it is important to comply with the following constraint:
                              L          RL                =                                                            R                ⁢                                                                  ⁢                                  1                  ·                  R                                ⁢                                                                  ⁢                2                                                              R                  ⁢                                                                          ⁢                  1                                +                                  R                  ⁢                                                                          ⁢                  2                                                      ⨯            C                    ⁢                                          ⁢          1                                    equation        ⁢                                  ⁢                  (          1          )                    
If the condition expressed by equation (1) is satisfied, VC1 is the image of the current in the inductor. The voltage VC1 across the terminals of C1 is given by the following relation:
                              V                      C            ⁢                                                  ⁢            1                          =                              (                                          R                ⁢                                                                  ⁢                                  1                  ·                  RL                                                                              R                  ⁢                                                                          ⁢                  1                                +                                  R                  ⁢                                                                          ⁢                  2                                                      )                    ·          I                                    equation        ⁢                                  ⁢                  (          2          )                    
The device of FIG. 1 therefore makes it possible to obtain the image of a current passing through an inductor on condition that the value of the intrinsic resistance RL of the inductor is known.
It may be pointed out that the image of the current is given by an equation of the type: VC1(I)=a.I with “a” the proportionality coefficient.
For certain applications, it is necessary that the measurement exhibit an offset voltage, that is to say that contrary to the above equation (2), when the current I is zero the voltage VC1 is not zero. The equation which conveys this behavior is of the form VC1(I)=a.I+b with:
“a” the proportionality coefficient and “b” the ordinate at the origin (offset voltage in our case).
To produce this offset in the measurement, it suffices to join an additional arrangement to the device of FIG. 1.
FIG. 2 shows a device for measuring the current I in the inductor 12 with a voltage offset Voffset.
In the device for measuring the current of FIG. 2, the voltage which is the image of the current in the inductor is no longer VC1, as in the device of FIG. 1, but becomes the voltage Vmes.
The measurement circuit of FIG. 2 furthermore comprises elements of FIG. 1, an offset circuit 14 having a DC voltage Vout generator E connected in parallel, with an offset resistor Roffset in series with two resistors in parallel R3 and R4. The positive pole of the generator being connected to the common point of the two resistors R3 and R4 and to the terminal B of the inductor 12, the negative pole of the generator being connected to the resistor Roffset. The voltage generator E may be the output capacitor of a converter, this being the case for a Buck type chopper, for example.
It is also possible to place a capacitor C′1 in parallel with R3 and R4 so as to balance the impedances on the two branches of the measurement device.
The voltage Vmes is measured between the common point ca between the two resistors R2 and R1 and the common point cb between the resistor Roffset and the two resistors R3 and R4 in parallel.
The expression for Vmes relating to the device of FIG. 2 may be written:
                    Vmes        =                                            (                                                R                  ⁢                                                                          ⁢                                      1                    ·                    RL                                                                                        R                    ⁢                                                                                  ⁢                    1                                    +                                      R                    ⁢                                                                                  ⁢                    2                                                              )                        ·            I                    +                      Vout            ·                          (                              1                -                                  Roffset                                                                                    R                        ⁢                                                                                                  ⁢                                                  3                          ·                          R                                                ⁢                                                                                                  ⁢                        4                                                                                              R                          ⁢                                                                                                          ⁢                          3                                                +                                                  R                          ⁢                                                                                                          ⁢                          4                                                                                      +                    Roffset                                                              )                                                          equation        ⁢                                  ⁢                  (          3          )                    
The voltage Vout being constant, the voltage Voffset is therefore also constant, the equation obtained is therefore of the form: Vmes(I)=a.l+b, with:
  a  =                    (                              R            ⁢                                                  ⁢                          1              ·              RL                                                          R              ⁢                                                          ⁢              1                        +                          R              ⁢                                                          ⁢              2                                      )            ⁢                          ⁢      and      ⁢                          ⁢      b        =          Vout      ·              (                  1          -                      Roffset                                                            R                  ⁢                                                                          ⁢                                      3                    ·                    R                                    ⁢                                                                          ⁢                  4                                                                      R                    ⁢                                                                                  ⁢                    3                                    +                                      R                    ⁢                                                                                  ⁢                    4                                                              +              Roffset                                      )            
This type of measurement is used in devices such as the switched mode converters of power electronics, for which it is necessary to limit the current passing through the inductors. For this purpose, the measurement voltage Vmes is compared, with the aid of a threshold-based comparator, with a threshold voltage Vthreshold corresponding to a maximum current Imax.
However the device of the prior art represented in FIG. 2, has a major drawback since the value of the resistance RL of the inductor depends on the temperature to which it is subjected, the current measurement which is obtained therefore exhibits an error related to the temperature. The consequence is that the measurement of the current in the inductor and therefore the limitation of current Ilim depends on the temperature.
FIG. 3 shows a curve of variation of the limitation current Ilim as a function of the temperature T of a current limiting circuit comprising a threshold-based comparator and the measurement device of FIG. 2. The variation in the limitation current as a function of the temperature is 60% between −40° C and 100° C. Such a scatter in the value of the limiting current gives rise to an oversizing of the power circuit so that it can withstand the limiting current at low temperature, this representing a major drawback.